Free-flowing pressure sensitive adhesives

Description

[0001] The invention relates to hot melt adhesives which are pressure sensitive adhesives whereby the adhesive is provided in a granulated form and is free-flowing and does not block during storage.

[0002] Thermoplastic adhesives, especially hot melts, can be manufactured and packaged as disclosed in EP 0469564. According to this document a hot melt adhesive is formed as separate portion, solidified and thereafter this portion is surrounded by a plastic packaging material. The plastic packaging material shall be meltable with the adhesive. The range of the pillow-shaped portions disclosed is from about 100 g to about 4 kg.

[0003] In order to reduce the tendency of the individual pillows to stick and adhere to each other, it is known to coat them with a separating, anti-stick substance, as disclosed in US 7328547. The anti-stick substance is disclosed as film forming material containing at least 25% wax. The adhesive can be manufactured for example in a co-extrusion process. No specific form of the adhesive as delivered is disclosed.

[0004] From EP 0957029 it is known to manufacture coated portions of an adhesive by co-extrusion. The polymer is provided in molten form and during the extrusion process an outward coating is applied. No information of specific compositions of the adhesive or coating material is provided.

[0005] EP 1196509 discloses a method to form granulated hot melt adhesives. This granulate is cooled to a non sticky form. Thereafter the adhesive material is coated with an ingredient of such adhesive in a liquid form which is non sticky as solid material and shall form an outside coating on such granulates. Such granulates a disclosed as free flowing.

[0006] The prior art teaches the possibility to coat larger portions of a hot melt adhesive with a plastic coating. Such coatings are required for packaging pressure sensitive adhesives which by way of their use shall provide permanent tacky surfaces. Only with such non-sticky coating the adhesive material can be transported in single portions.

[0007] The coating materials as disclosed in the prior art are selected so that they can be mixed with the adhesive and do not negatively impart the adhesive property of the adhesive. In case of a powdery coating material this is easy to apply. But for use of such portions it is required that this coating shall not be damaged during transportation. In case of defects of the shell the portions will adhere and the portions have to be separated before further processing. Also in case of elevated temperature during transportation and storage it has been noticed that coatings manufactured of powder are not stable and so the parts, blocks or pillows will stick together.

[0008] If the materials are coated with films of a thermoplastic polymer the stability of such coat is higher. But the manufacturing process is complex. As disclosed the packaging process provides pillows or blocks which are handled manually. An automatic feeding process of smaller portions to a melting device is not possible.

[0009] Coating of smaller particles is disclosed by using as coating a fluid which shall form a solid layer on the surface. This puts a strong limitation on the process, as only specific coating materials can be selected limited by the requirement of having a liquid coating as solution or dispersion. The coating process in a liquid form as dispersion is energy consuming. The solvent has to evaporate to get a non sticky surface. Such outer layer needs to be stable and non blocking even under more humid or warm conditions.

[0010] As the adhesive is handled in practice manually larger portions of the adhesive are preferred. Such forms of packaging have the disadvantage that a supply of smaller amounts to the application device is not possible. So the actual molten amount of an adhesive in a melting tank to maintain a steady supply of adhesive is relatively large, which leads to thermal stress of the adhesive melt.

[0011] Another disadvantage of smaller packaged adhesive portions is based on the form. The relationship of shell volume and core volume requires a higher amount of shell material. So the adhesive after being molten will contain a larger amount of film forming material stemming from the shell of the portions. This may negatively impart the properties of the adhesive.

[0012] Consequently there is a need for a supply of supply pressure sensitive adhesives (PSA) hot melt adhesives in form of pellets which shall be supplied in bulk form and can be processed by automatic feeder systems. But in spite of such requirements no PSA adhesive in form of pellets form is available in the market.

[0013] So it is a need for the adhesive manufacturer to PSA in a granulated form so that smaller amounts of adhesive can be supplied. Additionally the adhesive should be provided in a form which can be stored and thereafter automatically transported and fed into the application devices in an application site. The pellets shall be covered with a non-blocking coating which can be melted together with the adhesive without unfavourably influencing the properties of the adhesive. The coating film shall be homogenously mixable with the adhesive. Additionally the adhesive pellets shall be non-blocking during storage and transportation.

[0014] The objective of the invention is solved by providing a hot melt pressure sensitive adhesive consisting of an adhesive in form of pellets, such adhesive comprising at least one polymer selected from polyesters, polyacrylates, polyolefins, polyurethanes, polymers of ethylene/ vinylacetate (EVA), polystyrene copolymers or mixtures thereof, at least one tackifier and additives, each pellet of adhesive comprising an outer shell of a film forming material, whereby the adhesive having a softening point of 80 to 150°C, having a tacky surface at 25°C, the film forming material is selected from thermoplastic polymers having a melting point of less than 120 °C, being a non tacky material, each pellet being completely surrounded by the film forming material, so in that the pellets have a non-blocking surface and are free flowing at a temperature of less than 45°C.

[0015] The term pellets shall include particles of small size showing a symmetrical or unsymmetrical form, having a weight of up to 15g each. The size is less than 25 mm in either direction the lower limit is more than 1 mm. The pellets according to the invention are free flowing. Under free flowing it is understood that a bulk of pellets will be flowing under influence of the gravity through a perpendicular tube at an indicated temperature. The tube shall have a diameter of 10 cm and a length of 50 cm. The pellets are flowing, no clogging, sticking or blocking is observed. The temperature of pellets and tube is the same and is less than 45°C.

[0016] Many thermoplastic synthetic polymers can be used in appropriate hot melt adhesives. These polymers can be blended with other ingredients such as plasticizer, tackifier, oils and other additives, to form a pressure sensitive adhesive (PSA). As main polymer of an adhesive used in the invention all thermoplastic polymers can be selected which impart pressure sensitive properties to the adhesive and which are compatible with tackifiers, plasticizers and other components to form a substantially homogenous melt. The adhesive is solid at room temperature and is applied in a molten form. It is necessary that the adhesive remains homogenous for a period of time in the molten state.

[0017] A variety of available thermoplastic materials can be used in the adhesives. They are selected from thermoplastic polyesters, polyacrylates, polyolefins, polyurethanes, ethylene vinylacetate polymers (EVA), polystyrene copolymers or mixtures. Preferably these polymers are non-reactive. Examples of such thermoplastic polymers are elastic polymers, such as styrene block copolymers, like SIS, SBS, SEBS, SEPS, SIBS; ethylene/vinyl acetate polymers (EVA) and other ethylene copolymers, like ethylene/methacrylate, ethylene/n-butyl acrylate and ethylene/acrylic acid polymers; polyolefins, such as polyethylene, polypropylene and copolymers, amorphous poly-α-olefines (APAO); polyvinyl acetate and copolymers; poly(meth)acrylates from different acrylic monomers, for example alkyl(meth)acrylates, hydroxyalkyl(meth)acrylates, acrylamides; polyesters; polyvinyl alcohols; polyurethanes; copolymers of vinyl monomers and polyalkylene oxide.

[0018] Such polymers are commercially available in different compositions and molecular weights. The man skilled in the art can easily select useful thermoplastic polymers according to their melting point, viscosity or adhesive properties in accordance to the required purpose of the adhesive.

[0019] Such PSA shall contain additionally a tackifying resin in combination with a thermoplastic polymer and optionally a plasticizer and other components.

[0020] Tackifying resins useful in the adhesives of the invention comprise natural and modified resins, polyterpene resins, phenol-modified hydrocarbon resins, aliphatic and aromatic hydrocarbon resins, hydrogenated hydrocarbons, hydrogenated resins and hydrogenated resin esters and rosins.

[0021] Examples for rosins and its derivatives include wood rosin, tall oil, colophonium, gum rosin, wood rosin, rosin ester resins including its esters, hydrogenated or dehydrogenated forms; terpene resins include for example natural and synthetic terpenes, polyterpenes and terpenesters; aromatic or mixed aliphatic-aromatic tackifying resins, like polymers from cyclopentadiene, dicyclopentadiene; styrene resins, like copolymers from styrene, alphamethyl styrene, vinyl toluene, methoxy styrene, tertiary butyl styrene, chlorostyrene; aliphatic resins from monomers 1,3-butadiene, cis-1,3-pentadiene, trans-1,3-pentadiene, 2-methyl-1,3-butadiene, 2-methyl-2-butene and other co-polymerisable monomers or aliphatic petroleum hydrocarbon resins.

[0022] It is preferred if the tackifying resins have a softening point from about 80°C to 150°C (ring and ball method, measured by ASTM E28-58). The adhesive composition comprises an amount of resin between 10 to 60 % by weight (related to the adhesive composition).

[0023] Additionally a plasticizer can be added to the PSA. The plasticizer is preferably used for viscosity adjustment and is comprised in the hot melt adhesive in an amount of 0 to 25 wt.-%, preferably 5 to 20 wt.-%. Suitable plasticizers are medicinal white oils, naphthenic mineral oils, phthalates, adipates, polypropylene, polybutene, polyisoprene oligomers, hydrogenated polyisoprene and/or polybutadiene oligomers, benzoate esters, vegetable or animal oils and derivatives thereof.

[0024] Examples of plasticizer include hydrogenated plasticizers like oils or oligomers of polybutene. Also monohydric or polyhydric alcohols with a molecular weight of 1000 to 6000 g/mol may also be used, for example polyalkylene glycols. Another group of suitable plasticisers are esters, including, for example, liquid polyesters and glycerol esters, such as glycerol diacetate and glycerol triacetate as well as neopentyl glycol dibenzoate, glyceryl tribenzoate, pentaerythritol tetrabenzoate and 1,4-cyclohexanedimethanol dibenzoate. Also plasticizers based on aromatic dicarboxylic acid esters can be used, for example the esters of phthalic acid, isophthalic acid or terephthalic acid. Also esters of sulfonic acids are used as plasticiser. Fatty acids are also suitable plasticizers. Such components are commercially available.

[0025] Optionally waxes in quantities of 0 to 20 wt.-% by weight can be added to the PSA. The quantity is selected so that, on the one hand, the viscosity is reduced to the required range and, on the other hand, adhesion is not adversely affected. The wax may be of natural or synthetic origin. Suitable natural waxes are vegetable waxes, animal waxes, mineral waxes or petrochemical waxes. Suitable chemically modified waxes are hard waxes, such as montan ester waxes, sarsol waxes, etc. Suitable synthetic waxes are polyalkylene waxes and polyethylene glycol waxes. Preferred waxes are petrochemical waxes, such as petrolatum, microwaxes and synthetic waxes, particularly polyethylene waxes, polypropylene waxes, optionally PE or PP copolymers, Fischer-Tropsch resins, paraffin waxes or microcrystalline waxes.

[0026] In addition other typical auxiliaries and additives can be incorporated in a suitable hot melt pressure sensitive adhesive. Examples are stabilizers, antioxidants, light stabilizers and/or adhesion promotors. Their function is to protect the polymers against decomposition during processing, storage or use. The stabilizers, more particularly UV stabilizers or antioxidants suitable for use include phosphites, phenols, sterically hindered phenols of high molecular weight, polyfunctional phenols, sulfur- and phosphorus-containing phenols. Suitable compounds in the context of the invention are, for example, hydroquinone, hydroquinone methyl ether or phenothiazine. The selection and the properties are known to the person skilled in the art. They are added to the PSA in quantities of typically up to 3 wt.-% and preferably about 0.1 to 2 wt.-%.

[0027] The adhesive compositions can additionally contain other compatible polymers, fillers, pigments, dyes, oils, fragrances and other conventional additives. PSA type hot melts are generally known for example based on styrene block copolymers as disclosed in EP 0451920 or EP 1493794, acrylate type adhesives according to WO 02/010307, polyolefin type adhesives according to DE 19944225, EVA type adhesives disclosed in DE 102006054196. Such polymers shall have a softening point of about 80 to 150 °C, preferably up to 130°C, in particular less than 120°C.

[0028] The outside shell of the pellets is made by a film forming material comprising at least one thermoplastic elastomer, which is not tacky at ambient temperature at 25°C, preferably up to 45°C. The chemical composition of such film forming polymers can be similar to the polymers as mentioned above. But in contrast to the thermoplastic polymers of the adhesive composition it is the requirement that it should not possess tacky properties. As mentioned above the film forming polymer is selected from ethylene-based polymers, such as ethylene/vinyl acetate (EVA), ethylene acrylate, ethylene methacrylate, ethylene methyl acrylate, ethylene methyl methacrylate; high-density and low-density polyethylene; polyesters, such as polyethylene terephthalate, polybutylene terephthalate, thermoplastic polycarbonates; atactic poly-a-olefins, including polypropylene, atactic polypropylene and others; polyurethanes; styrene/acrylonitrile, styrene/butadiene rubbers, polybutadiene rubber; polyacrylates.

[0029] Examples of ethylene-based polymers are LDPE, VLDPE, LLDPE, MDPE obtained by ziegler natta catalyst polymerization, or LDPE, EVA, EAA, EMA, EBA obtained by gas phase random polymerization, or EO, EP, EB, EH, ESI copolymers, obtained by ziegler natta catalyst polymerization or single-site catalyst polymerization or metallocene catalyst polymerization. Examples of block copolymers including styrene-butadiene (SB), styrene-butadiene-styrene (SBS), styrene-isoprene-styrene (SIS), styrene-isoprene (SI), styrene-ethylene-butylene-styrene (SEBS), styrene-ethylene-butylene (SEB) styrene-ethylene propylene-styrene (SEPS) and styrene-ethylene propylene (SEP). The film forming polymer has a melting point of 70° C to 130° C, preferably between 90 to 120° C (measured by DSC).

[0030] The film forming material can include also other additives and auxillaries preferably those which are required for processing of the material. One embodiment of the invention uses a film forming material which shall contain only small amounts of less than 15 wt-% (related to the film forming material) of low molecular weight materials like wax. Another embodiment of the invention uses a film forming material which shall contain 1 to 15 wt-% (related to the film forming material) of at least one plasticiser. Preferably such plasticiser can be selected from hydrocarbon oils like paraffinic, naphthenic and/or mineral oils. This may enhance the process to homogenise adhesive and film polymer.

[0031] The film forming material can be blended with the adhesive in molten form. This can be accelerated by mixing devices like static or dynamic mixers, by pumping or by the normal convection of the melting process. The film forming material is selected to form a non tacky surface and shall form a barrier against the environment.

[0032] The shell of the pellets is made from a different composition than the PSA. The film shall have a thickness of about 2 to 200 µm, preferably 10 to 100 µm. This is dependent from the size of the pellets. If the amount of film forming material film is too large in relation to the amount of PSA, the properties of the adhesive will decrease. If the film does not cover to whole surface or is too thin, the stability of the flowing properties of the adhesive pellets is not ensured. The amount shall include up to 12 wt-% of film forming material, preferably about 1 to 10 wt-%, in particular more than 3 to 10-wt% (related to the adhesive composition).

[0033] The PSA according to the invention shall be provided in form of pellets or granulates, so that the material is free flowing. The pellets shall have a size of less than 25 mm in each direction, preferably less than 20 mm. The lower limit is more than 1 mm, preferably more than 3 mm. The shape of the pellets can be varied according to the manufacturing process. They can have the form of small pillows, preferably a spherical form like balls, or in another preferred embodiment they form a cylindrical shape. In such case the dimensions are different in each direction, for example in one direction 25 mm with a diameter of 2 to 10 mm. The form of the pellets is not required to be regular, e.g. a spherical form can be compressed or stretched, rods can be symmetrical or have a non regular form as long as the size of the granulates is not too large. The form will be influenced by the manufacturing process, for example the pellets are squeezed, cut and separated to give a partly round form. It is possible but not preferred to mix different forms and sizes of the pellets. Another aspect of the invention is related to the size of the pellets. Typically the pellets shall have a weight of less than 15 g each, preferably less than 10 g most preferred less than 5 g. Smaller particle size increases the flowing properties of the material. The pellets shall be completely covered by the film. It is necessary that all sides of the pellets are covered otherwise the stability of the non blocking behaviour is reduced.

[0034] The pellets according to the invention show free flowing properties at a temperature up to 45°C. This free flowing property shall remain stable also after storage at elevated temperature. This stability can be tested according to the following method. A 10 cm wide tube with plastic or metal walls is filled with the pellets up to a height of 10 cm. On top of the material a plate with a 500-gram weight is placed. The test device is then placed in a 35°C oven for 7 days. After this period the coated pellets were tested. They are removed from the oven, cooled to room temperature (about 20°C) and poured in the testing device for flowing properties.

[0035] A preferred method to manufacture such pellets is the co-extrusion process. In a co-extrusion process, the plastic material to be packaged is generally melt blended in a mixer and then squeezed or extruded through an appropriately sized orifice in a die while still at a temperature above or very close to the softening point of the material. The orifice and die may be of any conventional configuration and generally is such as to provide either a slot like or cylindrical like configuration for the plastic material as it is pumped through the orifice. The temperature of the die must be maintained above the melting point of the plastic material, and is typically in the range of 100° C to 150° C. In co-extrusion the polymeric film forming material is then simultaneously extruded from the die to surround the PSA to be packaged and thus forms an outer shell which encloses the adhesive to be packaged. Co-extrusion techniques are well known in the art, and suitable equipment for co-extrusion processes are known to a person skilled in the art.

[0036] As known, since the materials being extruded at elevated temperatures, the co-extrusion process typically occurs under water or is immediately immersed to water so that the plastic material and film that surrounds the material begins to cool immediately after being extruded from the die. Thereafter, the plastic material covered by the polymeric film is allowed to cool either in a water bath or in a refrigerant medium such as chilled glycol, liquid nitrogen, compressed carbon dioxide or the like, or under ambient conditions so that the wrapped adhesive is sufficiently cooled for handling. The coated adhesive will be pinched mechanically preferably after cooling to a more solid form into pellets of desired size, shape, or weight. By this process it is essential, that the separating step also covers the cut part of the granulate particles completely with a coat. Otherwise a long term stability of the flowing properties of the pellets is not secured. In a preferred embodiment of the invention it is possible to manufacture the pellets by a co-extrusion process where by the softening point of the film material is less or equal to the softening point of the adhesive.

[0037] The pellets may be further packaged in a container, box or bag for transportation. The container can be made of mechanically stable material so that large amounts of such pellets can be handled. The content of the container can be discharged by known means. After discharging the pellets it can be transported by the known devices, e.g. screws, belt conveyors and other systems. But preferably the pellets can be transported by air or vacuum for example in a vacuum feeder system.

[0038] Another aspect of the invention is the use of such pellets of PSA which are covered by a non-blocking film in automated supply operations of an application device. The pellets of the hot melt adhesive can be manufactured by co-extrusion process. The pellets are non sticky and free flowing and can be handled for further processing on belts, screws or pipes. They can be stored and transported in large packaging containers like bags, drums or tanks. During further processing at the customer's site a portion of the materials can be discharged from the packaging container and transported to the melting device. This transportation can be performed by known devices preferably by a vacuum feeder. In the melting tank the material is liquefied and than it can be pumped to the application devices for example nozzles or rolls.

[0039] As the pellets have a small volume for each particle, it is possible to feed the adhesive in smaller portions to the melting device. So the amount of PSA to be processed in the melting tank can be reduced. An automatically operating feeder system is possible. No manual feeding of the melting tank is required. The amount of adhesive can be easily metered by known methods and devices.

[0040] As only low amounts of pellets are fed in the melting tank, the melting process can be accelerated. The stock of molten material can be reduced. A degradation process of the adhesive during melting can be reduced.

[0041] The advantages of the adhesives according to the invention are the free flowing properties which allow a highly automated processing of the pellets. Another advantage is the improved storage stability of the pellets also at elevated temperature. So the PSA can be supplied to a customer in a bulk form. Additionally the handling of the adhesive is improved to allow transportation in a vacuum feeder or by pressurized air. The adhesive can be supplied in small portions a will be subject to less degradation in the melting and application process.

Claims

1. Hot melt adhesive consisting of a pressure sensitive adhesive comprising at least one polymer selected from polyester, polyacrylate, polyolefin, polyurethane, ethylene vinyl acetate polymers, styrene blockcopolymers or mixtures, at least one tackifier and additives the adhesives having the form of pellets, each pellet of adhesive has an outer shell of a film forming material, whereby

a) the adhesive has a softening point of 80 to 150 °C and has at 25°C a tacky surface,

b) the film forming material is a thermoplastic polymer with a melting point of less than 120 °C,

c) each pellet being completely surrounded by the film material,

so that the pellets have a non-blocking surface and are free flowing at a temperature of less then 45°C.

2. Hot melt adhesive according to claim 1, wherein the pellets have a weight of less than 15 g each.

3. Hot melt adhesive according to claim 2, wherein the pellets have a cylindrical or a spherical shape.

4. Hot melt adhesive according to claim 1 to 3, wherein the pellets have a diameter of less then 25 mm in each direction

5. Hot melt adhesive according to claim 1 to 4, wherein the outer shell has a thickness of 2 to 200 µm.

6. Hot melt adhesive according to claim 5, wherein the amount of film forming material is less than 12 wt-% of the adhesive composition.

7. Hot melt adhesive according to claim 1 to 6, wherein the melting point of the film forming material is equal or lower then the softening point of the adhesive.

8. Hot melt adhesive according to claim 1 to 7, wherein the film forming material is selected from polyethylene, polypropylene, polyester, polyacrylate, ethylene vinyl acetate polymer, styrene block copolymers or blends optionally containing up to 15% of at least an oil and/or a wax.

9. Hot melt adhesive according to claim 1 to 7, wherein the pellets are manufactured by a co-extrusion process.

10. A process to supply a melting device with a hot melt adhesive whereby a hot melt pressure sensitive adhesive according to claim 1 to 9 is supplied as bulk material, the pellets are discharged and the pellets are transported by a vacuum feeder to a melting device.

11. Use of a hot melt adhesive according to claim 1 to 8 in pellet form as pressure sensitive adhesive.

12. Use according to claim 10 in automated operated feeders, preferably in vacuum operated feeders.